1. Field of the Invention
This invention pertains to drive motors such as are used in disc drives and like devices. More particularly, this invention concerns those drive motors which operate off DC sources and have external rotors which can support one or more discs in a disc drive.
2. Description of the Prior Art
A prior-art device of this type is now commercially available, and is denominated a "Sextant Brushless DC Disk Drive Spindle", and is available from Rotron Inc. as Model 4700. This device has a rotor with four poles and a stator comprising a multiplicity of overlapping coils.
This known device has certain disadvantage. From a physical standpoint, constructing the stator and the stator winding is a complicated matter. Since the individual coils of the stator winding overlap each other at the axial faces of the stator, the coil ends are long and bulky. Deep slots in the stator are necessary in order to allow overlapping of the individual stator coils. As a result, when it is necessary to fill the grooves with copper, a difficult task is presented. The space factor will be undesirably low. Furthermore, there is not much room available in the center of the stator into which a journal for the rotor may be introduced.
From an electrical standpoint, other disadvantages exist. The total resistance of the stator winding is relatively large, and at high drive motor voltages the overlapping between the individual stator coils can cause isolation problems to exist. Additionally, the motor constant ##EQU1## (where k.sub.E is the ratio of voltage induced in the stator winding to the angular frequency of the rotor and R is the total resistance of the stator winding) is low, by virtue of the high R. Therefore, the prior-art device has a relatively low efficiency.
It would be advantageous to provide a drive motor of this type which would be easier to construct and which would have less bulky coil ends, in addition to a low resistance and a correspondingly higher efficiency. It would further be advantageous to provide a drive motor of this type in which interference problems caused by overlapping stator coils would not exist and which would allow to use a robust bearing system.
The invention of U.S. Ser. No. 06/060,879 could be applied advantageously in connection with this present invention and therefore is requested to be dealt with as part of it in combination. The benefit of such a combinatory use would result in a reduction of the magnetic stray-field flux upon neighbored heads.